It's not if you need to connect to an exchange server. It's for business users. You can still connect to exchange servers without the 45$ plan. Any user who isn't getting it through their business accounts should only need the 30$ plan. That is everything i've ever been told. If any rep wants to chime in, they are more then welcome.
I can confirm this as I am connected to my Exchange server at work with the $30 data plan.
The $45 data plan is for people ON business accouts. Not sure how or why they charge so much extra a month for "corporate accounts" but they do.
But like I said Exchange server works on the $30 plan as does everything else. No need to get the $45 data plan.